Various catalysts have been proposed for purifying the exhaust gas emitted from internal combustion engines by reducing harmful components contained in the exhaust gas such as hydrocarbons (HC), nitrogen oxides (NOx) and carbon monoxide (CO).
Catalyst composites comprising a precious metal dispersed on one or more refractory metal oxide supports are known for use in treating the exhaust of internal combustion engines to reduce nitrogen oxides, hydrocarbons and carbon monoxide gaseous pollutants. Such catalyst composites have been generally contained in units such as honeycomb monolithic carriers, and the catalyst composites are called catalytic converters or catalyzers, which are placed in the exhaust flow path from internal combustion engine systems to treat the exhaust before it vents to the atmosphere. Typically, the catalyst composites are formed on ceramic or metallic substrate carriers (such as the flow-through honeycomb monolith carrier, as described herein below) upon which one or more catalyst coating compositions are deposited.
Platinum (Pt) is an effective metal for oxidizing CO and HC after high temperature aging. Despite the lower cost of Pd compared to Pt, Pd-based catalyst composites typically show higher light-off temperatures for oxidation of CO and HC, potentially causing a delay in HC and/or CO light-off. “Light-off” temperature for a specific component is the temperature at which 50% of that component reacts. Despite the lower cost of palladium as compared to platinum, Pd-containing catalyst composites may poison the activity of Pt to convert hydrocarbons and/or oxidize NOx and may also make the catalyst composite more susceptible to sulfur poisoning when used in diesel engine exhaust systems. These characteristics have typically prevented the use of Pd as catalyst composites in lean burn operations, especially for light duty diesel applications where engine temperatures remain below 250° C. for most driving conditions.
When palladium is used as the precious metal in the catalyst composite, alumina commonly used as the Pd support in treating emissions from both gasoline and diesel internal combustion engines. In addition, zirconia has been used as a palladium support, however, a significant loss of zirconia surface area has prevented the widespread use of zirconia supports for this purpose. Such supports suffer from lack of hydrothermal stability. There is an ongoing need to improve the performance of such catalysts, and an improvement of 5% reduction in residual hydrocarbons, carbon monoxide and nitrogen oxides is considered an excellent performance improvement.
As emissions regulations become more stringent, there is a continuing goal to develop catalyst composites that provide improved performance, for example, light-off performance and removal of residual hydrocarbons, carbon monoxide and NOx. There is also a goal to utilize components, for example, palladium, as efficiently as possible.